The present invention relates to turbomachines and similar apparatus having components formed of silicon nitride ceramic material. More specifically, the present invention relates to protective coatings for such components.
Certain components of gas turbines are often coated or even formed from silicon nitride ceramic material to counteract the adverse effects of oxidation and water vapor that can occur at extremely high operating temperatures. Such components may include the turbine blades, rotors, and nozzles. By forming turbomachine components of silicon nitride, it becomes possible to operate the turbomachines with improved efficiency and reduced pollutant emissions at temperatures above 1200xc2x0 C. However, for operating temperatures approaching 1480xc2x0 C., even silicon nitride blades, rotors and nozzles can be adversely effected by oxidation and water vapor present in the flow stream.
It would be desirable to protect the silicon nitride substrates of the components in a way that reduces oxidation and also provides a thermal barrier. Partially stabilized zirconia (PSZ) has been employed as a thermal barrier coating for components made of superalloy. However there is a significant mismatch in the thermal expansion coefficient between zirconia (xcx9c10xc3x9710xe2x88x926) and silicon nitride (xcx9c3.5xc3x9710xe2x88x926). Because of this thermal mismatch, thermal cycling can easily debond the coating from the silicon nitride substrate. In addition, zirconia is a fast ionic conductor for oxygen and a poor oxidation barrier.
Though silicon nitride blades and nozzles have exhibited excellent mechanical and thermal properties, even when employed in flow streams at temperatures in excess of 1200xc2x0 C., silicon nitride is not without its drawbacks. One drawback is impact failure, generally caused by foreign objects such as carbon or metallic particles encountered in the turbine engine flow stream. Because of this drawback, there has been some reluctance to even employ silicon nitride turbine components.
To overcome the low impact resistance problem, it would be desirable to somehow strengthen the silicon nitride component substrate to enhance the impact resistance without adversely affecting the thermal properties of the substrate. It would also be desirable to minimize overall component weight.
The present invention is directed to machine components having substrates formed of silicon nitride. The substrate is coated with an interlayer of silicon nitride having a porous fibrous surface with a density in the range of about 85-98%. An outer layer of tantalum oxide or similar compound having a thermal coefficient in the range of 2.5xcx9c5.0xc3x9710xe2x88x926 may cover the interlayer.
Another aspect of the invention is directed to a process for coating turbomachine components with a multi-layered coating. The method includes the step brushing or spraying an interlayer of a silicon nitride powder slurry onto the component substrate. The slurry and substrate composition are then sintered at a temperature in excess of 1700xc2x0 C. to grow porous fibers on the composition. The composition is then infiltrated with a precursor solution which may include a pre-ceramic polymer. The composition is heated to convert the precursor into a matrix surrounding the fibrous silicon nitride layer in order to protect the component during operation. The interlayer has a density in the range of 90-98%.
An outer layer of tantalum oxide may be applied to the outer surface of the interlayer by Electron Beam-Physical Vapor Deposition.